In communications context, facsimile machines and some computerized data sources operate at data rates of 2.4 k bps for the data, and may have associated control information. Such information must be transmitted to the desired destination for use.
The data path may have an error-prone characteristic, so error detection and correction bits must be added. The data path is also bandwidth-limited. FIG. 1 is a simplified block diagram illustrating a communication system 10 for communicating data/fascimile signals between a control-plus-2.4 kbps data "digital terminal equipment" (DTE) source 12 and a called party 40. Communication system 10 of FIG. 1 includes a data path extending between antennas 20 and 22 which is part of a prior-art GSM cellular communication system, which uses existing standards for a GSM cellular communication system, as described in more detail below.
The GSM system is described in detail in the text The GSM System for Mobile Communications, subtitled A Comprehensive Overview of the European Digital Cellular System, authored by Michel Mouly and Marie-Bernadette Pautet, and published in 1992 by the authors, at 4, rue Elisee Reclus, F-91120 Palaiseau, France. Another text that describes the GSM system is Mobile Radio Communications, by Raymond Steele, published by Pentech Press, London, ISBN 0-7273-1406-8.
In FIG. 1, source 12 transmits or sends data at 2.4 kbps on a data (D) line or path to a rate adapter 14, which converts the data to 3.6 kbps. In the GSM system, rate adapter 14 is known as RA1'. Source 12 also sends asynchronous control information over a C path to rate adapter 14. Rate adapter 14 multiplexes the signals from the D and C paths, and adds auxiliary information bits known as "E4, E5, and E6" bits, which represent information relating to the network independent clocking. In simple terms, network independent clocking involves clocking information to take into account differences between the data rate of source 12 and the remainder of the system of FIG. 1. The multiplexing of the auxiliary information bits with the control bits and the 2.4 kbps data bits in rate adapter 14 brings the output bit rate to 3.6 kbps, organized into successive 36-bit, 10 millisecond (msec.) frames. FIG. 2 illustrates the data bit organization of a typical 10 msec. frame. In effect, rate adapter 14 maps the information, including data and control bits, from the 2.4 kbps data rate to the 36-bit, 10 msec frame of FIG. 2.
In the data organization illustrated in the frame of FIG. 2, there are 24 bits designated "D", which are data bits. More particularly, there are 24 D data bits, denominated D1, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, D12, D13, D14, D15, D16, D17, D18, D19, D20, D21, D22, D23, and D24. In addition to the D bits, the frame of FIG. 2 includes two bits designated X, five status bits designated S1, S3, S4, S6, and S9, and four auxiliary bits designated E4, E5, E6, and E7, for a total of 36 bits. In the 10 msec., 36-bit frame of FIG. 2, time proceeds from left to right, top to bottom, so the first-occurring bit is that one designated Di, and the second bit is D2. The last bit in the 10 msec. frame is S8. In the frame of FIG. 2, a rudimentary form of error protection is available in the form of duplication of the X bit.
The 3.6 kbps data stream represented by successive 10 msec., 36-bit frames, such as that shown in FIG. 2, is further processed in processing (PROC) block 16 of FIG. 1 by addition of error detection and correction (EDAC) codes, for transmission by a radio 18 and associated antenna 20 over an air link of the GSM system. The signal leaving processor 16 has a data rate of 11.4 kbps for a transmission over a half-rate-channel, or 22.8 kbps for a full-rate GSM channel. Those skilled in the art know that radio 18 performs modulation and upconversion as needed. At the receiving end of the air link, antenna 22 receives the signal, and routes it to a radio 24, where it is downconverted and demodulated, as may be required to regenerate the 11.4/22.8 kbps data. A processor 26 performs error correction and detection, as by use of Viterbi decoding, and produces what is expected to be error-free data at 3.6 kbps, a replica of, or equivalent to the data at the output of block 14, representing the multiplexed data, synchronization, and control. The 3.6 kbps data and control signals from block 26 ae applied to a demultiplexer 28, where the signals are demultiplexed to control (and any synchronization signals which accompany the control), together with 2.4 kbps data. The data stream, demultiplexed by block 28 into data, control, and auxiliary information portions, is transmitted to a second rate adapter 30 by way of a D data path, and the control and auxiliary information are transmitted by way of a C path.
The 2.4 kbps data and the control signals from demultiplexer 28 of FIG. 1 are applied to a second rate adapter 30, which is designated RA1'/RA1 in the GSM system. Second rate adapter 30 maps the 2.4 kbps data and its accompanying control signals into an 8 kbps data stream organized into 10 msec., 80-bit, V.110 frames, as defined by the Consultative Committee for International Telephone and Telegraph (CCITT), now the International Telecommunications Union (ITU). FIG. 3 illustrates the data organization of V.110 ITU frames.
In the ITU V.110 10 msec., 80-bit frame of FIG. 3, time proceeds from left to right, top to bottom, as in the case of FIG. 2, so the first eight (synchronizing) bits of the frame are binary 0, and the 9th, 17th, 25th, 33d, 41st, 49th, 57th, 65th, and 73d (synchronizing) bits are binary 1. In order to form the frame of FIG. 3 from the 2.4 kbps data stream emerging from demultiplexer 28 of FIG. 1, the synchronizing binary 0 bits are first inserted in bit positions 1 to 8, and the binary 1 bits are inserted into the abovementioned 9th, 17th, 25th, 33d, 41st, 49th, 57th, 65th, and 73d positions. As in the case of FIG. 2, the bits of FIG. 3 designated D represent data bits, identified by a suffix ranging from 1 to 24, relating to the individual ones of the 24 data bits. As can be seen in FIG. 2, all of the data bits D are duplicated. Since both the 36-bit frame of GSM and the 80-bit V.110 frame have a duration of 10 msec., they are associated on a one-to-one basis, although there may be a time difference between their occurrence. That is, the information from a "current" 36-bit frame is available to populate the 80-bit frame, by, for example, for each data (D) bit, inserting bit value in the two appropriate locations in the V.110 frame. Note that the E1, E2, and E3 bits of the V.110 frame do not appear in the 36-bit frame. The values of E1, E2, and E3 represent the source data rate, which is available during set-up of a GSM call, but does not appear in each 36-bit frame. Thus, the values of E1, E2, and E3 can be stored in rate adapter 30 of FIG. 1 at the time the call is initiated, and saved for insertion into each later frame.
The 8 kbps data flow represented by the successive V.110 frames outputted from second rate adapter block 30 of FIG. 1, and as represented by FIG. 3, is further increased in data rate in a third rate adapter block 32 to 64 kbps, by filling with binary ones, for use by ISDN-compatible devices. This third rate adapter is known as RA2 in GSM and in ISDN literature. From third rate adapter 32, the 64 kbps data is applied to data circuit equipment (DCE) 36, which is a part of mobile switching center (MSC) 34, which is IDSN-compatible. MSC 34 in turn connects to a public switched telephone network (PSTN).
MSC 34 of FIG. 1 contains other DCEs 36 for carrying other calls, and also performs other switching functions. DCE 36 is a modem for interfacing with a network 38, which may be, for example, a public switched telephone network (PSTN), an ISDN network, or a private network. The data applied to DCE 36 is modulated in a manner suited for transmission over network 38, and is routed over network 38 to the called party 40. At the called party 40, a modem or DCE 42, corresponding to or interfacing with DCE 36, converts the modulated signal into 2.4 kbps data and control signals.
A block 44 represents a data sink or facsimile machine for using the data originating from DTE 12. Of course, while the arrangement of FIG. 1 has been described as transmitting information from DTE 12 to DTE 44, transmission may be accomplished in both directions over the same channel.
In a spacecraft or satellite communication system, it may be desirable to use GSM standards for compatibility, to the extent possible. Thus, signals at 2.4 kbps from DTE 12 of FIG. 1 must be processed for transmission over satellite air links, rather than for terrestrial air links. Ideally, the same processing would be used as in GSM. However, the satellite communication link differs from the GSM terrestrial air link in a number of ways, particularly in the values of transmitted power and bandwidth. The rates at which data is transmitted over the satellite communication links therefore differ from those of GSM. FIG. 4 illustrates, inter alia, a satellite communications system known as ACeS.
A full-rate channel of the ACeS satellite communication system, for example, is expected to have a data rate of 24 kbps; this corresponds to the 22.8 kbps rate at the output tof block 16 of FIG. 1. While this 24 kbps is greater than the 22.8 kbps of the GSM full-rate channel, most of the ACeS services are offered on a quarter-rate channel (6 kbps), while most GSM services are offered on a full-rate channel (22.8 kbps). Thus, a 2.4 kbps data call or link, such as that described in conjunction with FIG. 1, would, in a satellite context, require a 3.6-to-6 kbps rate conversion for error correction, rather than a 3.6-to-22.8 kbps rate conversion. In general, the link margin is less on the satellite system than on the GSM terrestrial systems. The margin for error correction is only 6/3.6 in the satellite context, by comparison with the 22.8/3.6 margin in GSM. This lesser margin is considered to be insufficient for commercial use.
Consequently, some other method must be used for rate adaptation.